Although substantial strides have been made in recent years in the bonding of wood particles such as shown in the Stofko U.S. Pat. No. 4,944,823, which relates to the use of isocyanate based adhesives, wood particles are still mostly bonded by liquid adhesives produced by the condensation of formaldehyde with urea, melamine or mixtures thereof, or with phenol, resorcinol or mixtures thereof.
Procedures have also been developed to convert liquid phenolic resins to water-soluble solid powders by spray drying. Such spray-dried phenol-formaldehyde resins in powdered form are more economical than liquid phenol-formaldehyde resins in the manufacture of some composite wood products. Thus, such powdered phenol-formaldehyde resins are being manufactured and used in the oriented strand board industry. Spray drying, however, is not suitable for producing powdered resins from other liquid resins, such as urea-formaldehyde or other aldehyde condensation products.
As was the case in 1985 when the application which matured into Stofko '823 was filed, there is still a continuous incentive to reduce the costs involved in the manufacture of bonded wood products. Moreover, it would still be desirable to avoid the use of formaldehyde condensation products in view of the fact that such condensates can release formaldehyde which is at least an eye and mucosa irritant, which is carcinogenic, and which certainly has an undesirable odor. Many attempts have been made to solve these problems. One approach is disclosed in an earlier Stofko patent, namely U.S. Pat. No. 4,183,997, which discloses a binder for wood based on carbohydrates and catalyst.
The use of liquid isocyanate adhesives such as 4,4-diphenylmethane diisocyanate (hereinafter MDI) has remained difficult for the reasons set forth in the aforementioned Stofko U.S. Pat. No. '823. The above-mentioned spray drying which can be used to produce powdered phenol-formaldehyde resins is also not suitable for producing powdered resins from MDI. Thus, a main problem of the use of MDI as a bonding resin is that it is too concentrated; as a result it is very difficult to distribute small quantities of MDI over a large surface of lignocellulose materials to be bonded. New liquid spraying techniques have improved but not solved the problem of resin distribution over the bonding surface. Most attempts to extend MDI, such as by emulsification and incorporation of water, have failed.
So far, the only practical means of extending MDI is that disclosed in Stofko U.S. Pat. No. '823 wherein the MDI is extended by a suitably active particulate material, exemplified therein as preferably carbohydrates, especially sugars and starches, by intermixing and pre-reacting the liquid MDI with such an active solid particulate material or pasty material. The reaction produces a more fully activated powdery adhesive, e.g. a carbohydrate powder, which can be more easily distributed than liquid MDI and which provides excellent composite wood products such as plywood, particleboard, fiberboard, chipboard and the like.
Whereas excellent composite products can be formed according to the exemplified embodiments of Stofko U.S. Pat. No. '823, the preferred active materials disclosed in that patent, namely sugars and starches, have the significant disadvantage of being soluble or gellable in water, and the inorganic active materials do not work as well. Moreover, the bulk densities of the MDI-reacted sugars and starches of U.S. Pat. No. '823 are undesirably high so that a much greater amount of powdered binder is needed for effective bonding of lignocellulosic materials than is desirable; for example, sucrose has a specific volume of 1.8 cm.sup.3 /g and wheat flour has a specific volume of 1.48 cm.sup.3 /g.
As indicated above, the preferred active carriers for the MDI in Stofko U.S. Pat. No. '823, i.e. starches and sugars, are relatively heavy. To obtain an easily spreadable powdery binder using starch or sugar according to Stofko U.S. Pat. No. '823, it is necessary to have a starch:MDI ratio of at least 4:1, or a sucrose:MDI ratio of at least 9:1; otherwise the binder is either pasty or even a high viscosity liquid due to the inevitable presence of moisture and the fact that sugar crystals are water soluble and starch particles are gellable in water. Thus, another problem according to the examples of Stofko '823, if insufficient starch or sugar is present, i.e. less than the aforementioned ratios, is that the binder is in a form which is difficult to use, namely the undesirable pasty or high viscosity liquid form which is difficult to use.
At the present time, there are only two industrially viable methods for distributing polymeric MDI-based adhesives, namely by spraying liquid adhesives or by high shear blending of powdered adhesives, and an easily spreadable powder consistency cannot be made according to the starch and sugar examples of Stofko U.S. Pat. No. '823 using lower ratios of starch or sugar to MDI than those indicated above, as would be desirable.
Thus, the need still exists for improvements in bonding materials for lignocellulosic materials such as wood chips, wood fibers and other organic materials including straw, organic fibers, etc., or even for woodboard in the manufacture of plywood. In particular, a suitable way to increase the efficiency of MDI distribution to a better level than what is disclosed in Stofko '823 has heretofore not been found.